1. Field of the Invention
The present invention relates to a method for inmold polymerization of an unsaturated compound having a vinyl or vinylidene group (hereinafter referred to as "vinyl type monomer"). More particularly, the present invention relates to a method for in-mold polymerization of a vinyl type monomer comprising mixing the vinyl type monomer optionally containing its polymer with a polymerization initiator and a polymerization accelerator in a specific manner and rapidly polymerizing and molding it in a mold to produce a molded article with good efficiency.
2. Background of the Invention
In general, a vinyl or vinylidene polymer (hereinafter referred to as "vinyl type polymer") is molded by melt molding, for example, injection molding comprising melting a polymer in the form of powder, beads or pellets at a high temperature and injecting molten polymer in a mold.
For producing a sheet material of polymethyl methacrylate, there are known a cell casting method which comprises pouring methyl methacrylate optionally containing its polymer into a cell made of glass plates and gradually polymerizing methyl methacrylate for a long period of time, for example, several hours and a continuous casting method comprising casting an unsaturated monomer comprising methyl methacrylate containing its polymer on an endless metal belt and polymerizing it in a comparatively short period of time (cf. Japanese Patent Publication Nos. 30985/1977 and 35393/1977).
When the vinyl type polymer is melt molded, for example, by injection molding, it should be molded in a narrow temperature range in which the polymer is molten but not thermally decomposed since the polymer tends to be thermally decomposed at a high temperature and pressure required for melt molding. Therefore, the melt molding of the vinyl type polymer requires a large, amount of energy and a large complicated apparatus. In addition, even at a high temperature, the molten vinyl type polymer has very high viscosity so that great skill is required to transfer surface images of the mold to the molded article precisely.
The cell casting is not generally employed except for molding the plate form article since a polymerization rate in this method cannot be increased, this resulting in poor productivity. Although the continuous coasting method has good productivity, it requires an expensive apparatus but can produce only a plate form article.
In these conventional methods, all the components including the monomer and the polymerization initiator are mixed together and poured in a reaction cavity to polymerize the monomer. In a case of a polymerization initiator providing a high polymerization rate, the polymerization reaction does not proceed homogeneously.